Thermoplastic polyurethanes (TPU) are of industrial importance in many applications because of combination of good mechanical properties with known advantages of inexpensive thermoplastic processing. By modifying components of TPU, a wide variety of properties can be achieved. Among all kinds of TPU, polyether-based TPU is one of the most important series because of its excellent hydrolysis resistance and low temperature flexibility. An overview of TPU including their properties and applications can be found in Hans-Georg Wussow, “Thermoplastic Elastomers”, Ullmann's Encyclopedia of Industrial Chemistry, Electronic Release, 7th ed., chap. 2 “Thermoplastic Polyurethane Elastomers”, Wiley VCH, Weinheim, 2004.
Soft thermoplastic polyurethane, which generally has a hardness of less than Shore A 98, in particular from Shore A 40 to Shore A 95, is hard to process due to small proportion of hard segment. In injection molding process, the soft TPU, especially soft polyether-based TPU, tends to stick in mold cavity and therefore needs a large mold releasing force (demolding force) to eject final product from mold. In the case of mass production of TPU products by injection molding process, it normally requires a fully automatic ejection of product from mold with a fixed demolding force. As a result, cooling time of soft TPU in mold must be prolonged in order to realize automatic ejection. In other words, the total cycle time to produce a final product by injection molding is therefore increased. The demolding force has become an important criterion to evaluate processability of a material in mass production through injection molding process.
It is known that some internal or external lubricant waxes e.g. amide wax or ester of montanic acids, as releasing agents can improve mold releasing behavior of TPU. An overview of lubricant waxes used in TPU can be found in H. Zweifel (Ed.), Plastics Additives Handbook, 5. Ausgabe, Hanser Verlag, Munchen 2001, P443ff.
The lubricant wax has been widely used in current TPU product. It is known that TPU with a large amount of an amide wax for example N,N′-ethylene bis-stearylamide (EBS) wax shows good mold releasing property. However, lubricant wax often leads to a strong surface blooming of final TPU product after injection molding, especially after annealing at a certain temperature or placing in a humid weather condition. TPU with ester of montanic acid, e.g. Licowax E from Clariant, also shows good mold releasing property when the ester is used in a large amount. However, transparency and haze of final product will be seriously influenced by addition of ester of montanic acid.
Normally, polyether-based TPU should be processed at a processing temperature (i.e., maximum melt temperature of TPU) of more than 200° C. in the injection molding to ensure a good product quality, such as low shrinkage and good surface appearance. In other words, if the processing temperature of TPU is less than 200° C., in particular less than 180° C., shrinkage and inhomogeneous surface of TPU product often happen. However, it is well known that when TPU is thermally processed at a melt temperature of more than 180° C., it will start thermal degrading. Especially if the material is not well pre-dried so that moisture remains in the material, the degradation of TPU will accelerate, and thus the mechanical properties of TPU products will be significantly reduced.
It is known that some plasticizers can be used to improve processability of TPU. EP 0134455B2 disclosed a soft polyether-based TPU with a Shore A hardness of lower than 80 comprising di(methoxyethyl)phthalate or phosphate ester, e.g. diphenyl cresyl phosphate, as a plasticizer. WO 2010/125009 disclosed a soft TPU comprising an ester of a tricarboxylic acid with one alcohol, e.g. acetyl tributyl citrate (ATBC), as a plasticizer. Hexanedioic acid 1,6-bis[2-(2-butoxyethoxy)ethyl]ester is also used as a plasticizer for polyether-based TPU. However, improvement of processability in particular demolding force in injection molding achieved by the plasticizers is not sufficient. Moreover, these plasticizers often lead to an increased abrasion loss and sometimes reduced mechanical properties of TPU through injection molding.
Therefore, it is desirable to find a processing auxiliary which can improve the processability, especially mold releasing force, of soft polyether-based TPU in injection molding process without such disadvantages as increased abrasion loss, and undesirably significant shrinkage and surface blooming of the final product.
Cyclohexane dicarboxylic acid alkylester is known as a plasticizer as described in CN100406428C, which is incorporated here as a reference.
WO 2008045637 discloses an in-situ production process for preparing a polyester TPU with a good low temperature snap back in presence of plasticizers, e.g., phthalate ester. It is mentioned that for applications that are sensitive from the toxicological point of view, such as children's toys and food contact, 1,2-cyclohexane dicarboxylic acid diisononyl ester may be used as the plasticizer.
US 2010/0286297 A1 discloses an application of cyclohexane dicarboxylic acid alkylester as an internal mold release agent for integral polyurethane foams.
JP2010163557A discloses a thermoplastic elastomer consisting of styrene type thermoplastic resin (A), a resin (B) selected from polyester type thermoplastic resin and polyurethane type thermoplastic resin, and cyclohexane dicarboxylic acid alkylester (C). It is said that the thermoplastic elastomer has excellent moldability, bleed-proof property, reduced stickiness and high adhesiveness.
However, the above-mentioned documents do not mention that cyclohexane dicarboxylic acid alkylester can improve processability of polyether-based TPU in injection molding process and meanwhile does not lead to increased abrasion loss and significant shrinkage and surface blooming of final product.